The production of nitrogen by means such as membranes or selective adsorption modules, in the two cases often called "on-site means", has been considerably developed in recent years, all over the world, complementarily to the conventional production by cryogenic means, for the following reasons:
these "on-site" means offer excellent security of supply; PA1 low production costs; PA1 the possibility of supplying, at very attractive costs, according to the applications in question, nitrogen of suitable purity, sometimes called "impure nitrogen", such that the residual oxygen concentration of the nitrogen can vary from several ppm (parts per million) to several %. PA1 To produce nitrogen of reduced purity (5% of residual oxygen for example), there will be used compressed air brought to a relatively high temperature (60.degree. C. for example). PA1 To produce nitrogen of high purity (1000 ppm of residual oxygen for example), there will be used compressed air brought to a relatively low temperature, often near ambient temperature or below. PA1 a) If the on-site nitrogen production system (which can be of the membrane or adsorption type) supplies only one user station, compressed air, from an air compressor, is first treated in a so-called treatment station comprising the steps of removing oil from the air, drying, filtering out the particles, and bringing to a desired temperature according to the above remarks. In a second stage, the obtained air, thus treated, is sent to a gas separation unit (of membrane or adsorption type) adapted to produce nitrogen required by the user station at the end of the line. PA1 b) The case of a "central separation unit" or "single generator". In this case, among all the required purities, it will be chosen to produce nitrogen of the highest purity for all the stations, at the risk of providing "excessive quality" at certain stations. The configuration in question comprises a central separation unit (including an air compressor, an air treatment station, and an air separation unit), nitrogen of the selected purity from this central unit being sent by the central line to the different local supply lines, supplying the various user stations at the end of the chain. PA1 It will be immediately apparent that this solution is a costly one, because the separation unit must perform enormous separation work because it will produce from the air nitrogen of high purity, to fulfill the requirement for the most exacting use. It is thus a cumbersome solution, not only as to the energy required, but also as to the required surface of the membrane when the separation technique used is membrane separation. PA1 c) The second solution: the case of "multiple units of a compressed air network". The configuration is according to a preexisting compressed air network. It comprises a central air compressor, from which issues compressed air through a central line, this line supplying first a supply line for a compressed air user station, then in parallel a bundle of local distribution lines supplying several nitrogen user stations whose nitrogen purity requirements differ. Each local line comprises an air treatment station and a gas separation unit (whether of the membrane or adsorption type) adapted to the purity requirements of the user station at the end of the line. PA1 d) The third solution sometimes used to solve this problem of multiple users/multiple purities under the name "autonomous multi units" is constituted in the following fashion: each line supplying a user station is autonomous, and comprises an air compressor, an air treatment station, and a local air separation unit adapted to the nitrogen requirements of the user station which is located at the end of the line, the nitrogen from each of these separation units being then directly sent to the corresponding user station. PA1 being more economical than the existing solutions (necessary energy balance, necessary membrane surface if this technique is utilized, investment cost); PA1 permitting upgrading conventional membranes of different productivity and selectivity existing in the same lot of manufactured membranes or in different lots (manufacturing variances); PA1 permitting operating under more satisfactory safety conditions on the upstream side of the membrane, when the membrane technique is selected for one of the steps, PA1 for membrane modules located in the central separation unit, an air temperature within the range of 20.degree. to 90.degree. C., preferably within the range of 40.degree. to 60.degree. C. PA1 when treatment centers are decentralized at the local lines, the gas will be treated before its arrival at the local membranes so that the temperature will be within the range -60.degree. C. to 90.degree. C., and preferably within the range 15.degree. C. to 50.degree. C.
In the particular case of membranes, it is known that the membrane will react differently according to the temperature of the gas to be treated: thus it is known that at a high temperature (90.degree. C. for example), the productivity of the membrane increases, but the nitrogen permeation also increases, which results in the worsening of the O.sub.2 /N.sub.2 selectivity of the membrane. In this context, operation will most often be conducted under the following conditions:
At present the following different situations are encountered:
The situation becomes complicated when it is necessary to supply "impure" nitrogen (non-cryogenically), simultaneously, to several user stations requiring very different nitrogen purities. For example 95% N.sub.2 (5% impurities) at a first station, 99% (1% impurities) at a second user station, and 99.9% (1000 ppm impurities) at a third station. The arrangements generally provided in this case are described below respectively at b), c) and d):
Again it will be easily seen that this arrangement is not optimal, to the extent that each separation unit treats compressed air, because of the differences of purity (often called the "gap") of oxygen between the entering air and the leaving nitrogen, which can be high, and in particular the necessary large membrane surfaces when membrane technology is used for one of the separation stations. The energy used here, however, is moderate, but this advantage is generally counterbalanced by obtaining a product nitrogen pressure which is often relatively low (3 to 4.times.10.sup.5 Pa) in this situation in which network air is available whose pressure is fixed by the usual compressed air requirements (6 to 7.times.10.sup.5 Pa ordinarily sufficing).
Here again, this configuration has substantial drawbacks, connected to the useless multiplication of a certain number of stations; it will be noted in fact that for this solution a drawback connected to high investment exists, more than a high cost of operation.
For each of the four configurations described above (a), b), c), d)), it will be seen on the other hand that the separation unit directly treating the air for transforming it to oxygen with, as needed, a high purity "gap", the mixture obtained on the upstream side of the membrane is highly superoxygenated air, which is not without safety problems for the handling of this material.